How Long Is a Second?

The question of how long a second lasts certainly seems obvious, but in reality, defining that ubiquitous unit of time is anything but simple. And times can change ... literally. Thanks to newly developed, ultraprecise atomic clocks, the definition of a second could be changing as soon as the 2030s.

The second as we know it was most recently defined about 70 years ago with the use of a cesium clock, which measures the back-and-forth motion of cesium atoms. When pulsed with microwave energy, the atoms oscillate 9,192,631,770 times every second. For decades, this has been the most precise second ever defined, but now that definition looks to become outdated. Atomic optical clocks use visible light and measure atoms such as strontium that "tick" a lot faster, thus offering a more precise reading than cesium clocks. This new generation of clocks is so precise that they would have lost less than two minutes if they had started ticking when the Big Bang occurred 14 billion years ago.

"You can think of it as equivalent to having a ruler with tick marks every millimeter, as opposed to a stick that measures just 1 meter," said Jeffrey Sherman, a researcher with the National Institute of Standards and Technology's Time and Frequency Division in Boulder, Colorado. In other words, it looks like Bob Dylan's lyrics ring true: "The times they are a'changing."

Time for trivia: